1. Field of the Invention
The present invention relates to an index device of a machine tool having a motor built therein and a method of manufacturing the same.
2. Description of the Related Art
Recent machine tools such as machining centers use rotating table type index devices including a so-called direct drive structure in which a drive motor is directly coupled to the rotating shaft (see, for example, Japanese Patent Application Laid-Open No. 2009-248242). A direct drive index device is described with reference to FIG. 1.
A rotating table 11 for placing a workpiece (not shown) thereon is attached to a shaft 10 of a motor M that is contained in a casing 13 for housing the motor of an index device. The motor M includes a stator 17 and a rotor 16. The stator 17 has a stator coil 18 wound around a case 14 of the motor at the inner peripheral side thereof. The rotor 16 is attached to the shaft 10 that is rotatably journaled at the inner side of the stator 17 by a bearing 20 attached to a bracket 12 and a bearing 20 attached to a bracket 19. The index device having the motor M built therein is fixed to the table of a machine tool.
As depicted in FIG. 1, a recess is provided on the inner peripheral surface of the casing 13 for housing the motor M of the index device, such that gap space 15 is created in conjunction with the outer peripheral surface of the case 14 of the motor M so as to facilitate fitting of the motor M. Air is filled in the gap space 15.
As described above, in an index device having a motor built therein for use in a machine tool, gap space, i.e., back clearance, exists between the outer periphery of the motor and the inner periphery of the motor housing casing for the sake of assembly. Since the gap space is filled with the air, the thermal conductivity is lower, which could cause heat generated during the operation of the motor to be retained within the motor, accelerating rise in temperature of the motor. The motor becomes susceptible to overheat due to acceleration of temperature rise of the motor, which may decrease the continuous rated power of the motor.
Generally, forced cooling of the motor is conducted by circulating coolant in the gap space between the outer periphery of the motor and the motor housing casing so as to suppress rise in temperature of the motor. However, this involves, for example, additional provision of piping and pumps for circulating the coolant, as well as a temperature conditioner for managing the temperature of the coolant, causing increase in complexity of the structure of the index device and thus the costs therefor.